US4435708A - Means for eliminating step error in FM/CW radio altimeters - Google Patents
Means for eliminating step error in FM/CW radio altimeters Download PDFInfo
- Publication number
- US4435708A US4435708A US06/291,854 US29185481A US4435708A US 4435708 A US4435708 A US 4435708A US 29185481 A US29185481 A US 29185481A US 4435708 A US4435708 A US 4435708A
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- signal
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
- G01S13/06—Systems determining position data of a target
- G01S13/08—Systems for measuring distance only
- G01S13/32—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
- G01S13/34—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
- G01S13/345—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal using triangular modulation
Definitions
- the present invention relates to radio altimeters of the FM/CW type. More particularly, it relates to means for eliminating step error in the output of the altimeters.
- Radio altimeters of the FM/CW type have become the preferred equipment for accurately measuring aircraft altitude during approach to landing. Most of such equipment in operation is generally of the form described in U.S. Pat. No. 3,341,849 for "Self-Calibrating, Self-Testing Radio Altimeter" by B. L. Cordry et al.
- a transmitter is linearly frequency modulated by a modulating voltage having a triangular waveform.
- a portion of the transmitted signal is mixed with the reflected signal received from the ground to produce a beat frequency signal.
- the difference between the frequency of the transmitted signal and the received signal, the beat frequency is proportional to the aircraft altitude and an output indication of the altitude is produced by counting the frequency of the beat signal.
- step error has been recognized in prior equipment and efforts have been made to minimize the effect of the error.
- the frequency of the modulating wave is varied slightly, or wobbulated, tending to average out the step error.
- a more direct way to eliminate step error would appear to be to gate-out or prevent beat frequency data from being processed by the frequency counter during the time the data is subject to error.
- analog type frequency counters as used in the altimeter of U.S. Pat. No. 3,341,849, cannot readily be controlled so as to process accurately intermittent bursts of frequency data.
- a frequency counter can be easily controlled so as to give creditable results when operating with intermittent bursts of frequency data.
- the obvious manner of control for a digital processor is to generate a blanking gate in synchronism with the modulating wave and to utilize the gate to inhibit operation of a digital beat frequency counter during those intervals at which the modulating wave is near peak value.
- Such means still present the possibility of error in altitude measurement corresponding to some fractional part of a beat frequency signal cycle since the period during which the beat frequency counter is enabled most probably will not be equal to an integral number of beat frequency cycles.
- Another object of the invention is to provide a radio altimeter in which frequency data indicative of altitude is processed digitally to produce altitude information in digital form.
- the invention comprises, in a radio altimeter, digital means for generating a triangular modulating wave for a frequency modulated transmitter.
- Digital means synchronized with the triangle wave generator produces a count gate which is at a high logic level during most of the linear portion of the triangle wave period and which is at a low logic level during the portion of the period of the triangle wave near the wave peaks.
- the count gate and the beat frequency signal produced by mixing transmitted and received signals, are applied to logic means which modifies the duration of the high level state of the count gate to produce a derived count gate having a high logic level always of such duration as to equal an integral number of cycles of beat frequency signal.
- the derived count gate is then used to control a beat signal frequency counter and a precision clock counter the outputs of which are arithmetically processed to yield digital altitude information free of step error.
- FIG. 1A is a waveform diagram showing the waveform of the modulation voltage and the relationship between the frequencies of the transmitted and received waves of an FM/CW radio altimeter;
- FIG. 1B is a waveform diagram showing a count gate, synchronized with the modulation waveform of FIG. 1A, which may be used for blanking-out erroneous information in the difference frequency signal of the altimeter;
- FIG. 1C is a waveform diagram representative of the difference frequency signal of the altimeter
- FIG. 1D is a waveform diagram of a derived count gate used to eliminate fractional count errors in counting the frequency of the difference frequency signal
- FIG. 2 is a functional block diagram broadly showing an FM/CW radio altimeter and the means of the present invention for eliminating step error from the output altitude indication.
- a triangular wave 10 is shown which represents the form of a modulating voltage applied to a frequency modulated radio altimeter transmitter as well as the frequency variation of the transmitted signal.
- a similar wave 11 represents the frequency variation of the signal received by the altimeter.
- the displacement ⁇ t between the waves 10 and 11 along the time axis is proportional to the altitude of the aircraft carrying the altimeter.
- F d the difference between the frequencies of waves 10 and 11 is constant through the major portion of the cycles of the waves.
- Step error appears, however, at times near the peaks of the waves, t 1 , t 2 and t 1 't 2 ' when the frequency difference drops from a constant value to zero and then becomes reestablished at the constant value at a time following the peak of the received wave.
- FIG. 1B shows a count gate synchronized with the triangular wave of FIG. 1A.
- the count gate is at a logic "1" level during times at which valid difference frequency signal F d is being received and the count gate is at logic "0" level at times surrounding the peak times t 1 , t 2 , t 1 't 2 ' of the triangular waves when F d is subject to step error.
- FIG. 1C is a representation of the difference frequency signal F d simplified to the extent that the frequency is normally much higher than shown and that no attempt has been made to show the variation in frequency resulting from step error.
- FIGS. 1B and 1C illustrate the most probable case wherein the phasing and duration of the count gate relative to F d is such that a frequency count taken during the time that the count gate is at a logic "1" level will not contain an integral number of cycles of F d .
- This source of error is eliminated by applying the count gate of FIG. 1B and the difference frequency signal F d of FIG. 1C to logic means to produce a derived count gate shown in FIG. 1D.
- the derived count gate does not rise to a logic "1" level until the leading edge of the first F d cycle appears after the count gate of FIG. 1B has risen to a logic "1" level, as at time t 3 .
- the derived count gate continues at a logic "1” level as long as the count gate of FIG. 1B remains at a logic “1” level and does not fall to a logic "0" level until the leading edge of the first F d cycle appears following transition of the count gate of FIG. 1B to a logic "0" level, as at time t 4 .
- the derived count gate of FIG. 1D is therefore always of such duration as to contain an integral number of cycles of F d .
- the derived count gate is utilized to control counters for the frequency of F d and for a precision clock signal to provide numerical information which is processed to yield digital information of the aircraft altitude, as hereinafter described with reference to FIG. 2.
- a radio altimeter of the FM/CW type comprises a continuous wave transmitter 15 which supplies signal to a transmitting antenna 16 located in an aircraft to project a radio beam towards the ground.
- the signal from transmitter 15 is linearly frequency modulated between a low frequency F L and a high frequency F H by a triangular wave applied to modulator 17 to produce a transmitted signal having a frequency which varies with time as shown by waveform 10 of FIG. 1A.
- the triangular modulating wave suitably having a frequency of 150 Hz, is generated by an up-down counter 18 which counts cycles from a 240 kHz clock 19 and a digital to analog converter 21.
- Counter 18 can therefore be set to count up from zero to the binary equivalent of 799 then down to zero again during which time the running count is continuously converted to an analog voltage by converter 21, thereby producing a 150 Hz triangular wave.
- the beam transmitted by antenna 16 and reflected by the ground is received by antenna 22 then mixed with a small portion of the outgoing wave from transmitter 15 in receiver mixer 23.
- the signal at the output 24 of receiver-mixer 23 is the frequency difference signal of FIG. 1C containing frequency variations due to step error.
- the count gate of FIG. 1B is generated by a binary counter 26 and a logic circuit 27.
- Counter 26 continuously accumulates count of the output cycles of clock 19 during each halfperiod of the triangular wave output of converter 21.
- Logic circuit 27 decodes the output of counter 26 to produce a logic "1" after counter 26 accumulates the binary equivalent of 120 and until counter 26 has accumulated binary count corresponding to 648, whereupon the output of logic circuit 27 drops to the logic "0" level.
- the waveform of the count gate therefore exhibits a low level for approximately 0.5 ms after the start of a modulation wave followed by a high level for approximately 2.2 ms and again dropping to low level for approximately 0.5 ms.
- the count gate is so related to the triangular modulating wave that the high level state of the count gate appears during the linear portion of the triangular wave and the low level state appears prior to and persists after the peaks of the triangular wave, being substantially centered thereon.
- Logic circuit 28 may suitably comprise a D-type flip-flop with the count gate from logic circuit 27 applied to the D input thereof and the difference frequency signal F d from output 24 applied to the clock input thereof.
- a D-type flip-flop has the property of transferring the state of the signal present at its D input to its Q output upon the appearance of a high level signal at its clock input.
- the derived count gate of FIG. 1D is therefore always of such duration as to contain an integral number of cycles of F d and it is positioned by the count gate relative to the triangular wave of FIG. 1A so as to be in a high level state only during the linear portions of the triangular wave.
- the derived clock gate present on output 29 is applied to the gate input of a first 16-bit binary counter 31 which receives the F d signal from output 24 on the clock input thereto.
- the derived count gate from output 29 is also applied to the gate input of a second 16-bit binary counter 32 which receives at the clock input thereto the output of a 400 kHz clock oscillator 33.
- the output of counter 31 is the binary count of the number of cycles of F d occurring during the time derived count gate is in a logic "1" state.
- the output of counter 32 is the binary count of the number of cycles of clock signal from oscillator 33 occurring during the time the F d cycle count is being collected.
- the numerical outputs of counters 31 and 32 are processed arithmetically by a microprocessor unit 33 to produce the digital value of the aircraft altitude which may be displayed on an indicator 34 or otherwise utilized in the operation of the aircraft.
- F d is the frequency of the difference frequency signal
- c is the velocity of propagation of radio waves
- dF/dt is the slope of waveform 10 in FIG. 1A.
- F d is determined by dividing the output N 1 of counter 31 by the product of the output N 2 of counter 32 and the period T c of the clock signals from oscillator 33. Therefore, the microprocessor needs merely to divide N 1 by N 2 and multiply the resulting quotient by a predetermined constant to produce the numerical value of the aircraft altitude. For the stated parameters, ##EQU4## Substituting for F d in equation (2) ##EQU5##
- the predetermined constant (5 ⁇ 10 3 ) is, of course, converted to its binary equivalent prior to insertion in the microprocessor 33 and the output of the microprocessor appears in binary form which may be converted to decimal for display or retained in binary for utilization elsewhere.
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- Engineering & Computer Science (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Signal Processing (AREA)
- Computer Networks & Wireless Communication (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Radar Systems Or Details Thereof (AREA)
Abstract
Description
Claims (6)
Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US06/291,854 US4435708A (en) | 1981-08-10 | 1981-08-10 | Means for eliminating step error in FM/CW radio altimeters |
FR8213848A FR2511159A1 (en) | 1981-08-10 | 1982-08-09 | IMPROVED FREQUENCY MODULATION AND MAINTAINED WAVE RADIOMETERS |
JP57139092A JPS5841372A (en) | 1981-08-10 | 1982-08-10 | Step error removing device in frequency modulation/continuous wave type radio altimeter |
JP1990099066U JPH0342388Y2 (en) | 1981-08-10 | 1990-09-25 |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US06/291,854 US4435708A (en) | 1981-08-10 | 1981-08-10 | Means for eliminating step error in FM/CW radio altimeters |
Publications (1)
Publication Number | Publication Date |
---|---|
US4435708A true US4435708A (en) | 1984-03-06 |
Family
ID=23122144
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US06/291,854 Expired - Lifetime US4435708A (en) | 1981-08-10 | 1981-08-10 | Means for eliminating step error in FM/CW radio altimeters |
Country Status (3)
Country | Link |
---|---|
US (1) | US4435708A (en) |
JP (2) | JPS5841372A (en) |
FR (1) | FR2511159A1 (en) |
Cited By (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4905009A (en) * | 1987-06-01 | 1990-02-27 | Kaman Aerospace Corporation | Distance measuring device |
EP0359911A2 (en) * | 1988-09-12 | 1990-03-28 | Daimler-Benz Aerospace Aktiengesellschaft | Radar altimeter |
US5325095A (en) * | 1992-07-14 | 1994-06-28 | The United States Of America As Represented By The United States Department Of Energy | Stepped frequency ground penetrating radar |
US5869747A (en) * | 1996-05-22 | 1999-02-09 | William H. Hulsman | Food container internal pressure analysis |
US20080074308A1 (en) * | 2006-09-27 | 2008-03-27 | Honeywell International Inc. | Method and system of improving altimeter accuracy by use of a separate peak return signal tracking |
US20100245160A1 (en) * | 2009-03-31 | 2010-09-30 | Kabushiki Kaisha Toshiba | Fmcw signal generation circuit |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4468638A (en) * | 1982-02-11 | 1984-08-28 | The Bendix Corporation | Linear sweep frequency modulator for FM/CW radio altimeter |
Family Cites Families (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR1369397A (en) * | 1963-05-20 | 1964-08-14 | Csf | Improvements to frequency modulation radio altimeters |
US3968492A (en) * | 1974-03-18 | 1976-07-06 | Rca Corporation | Adaptive parameter processor for continuous wave radar ranging systems |
US3974501A (en) * | 1974-12-26 | 1976-08-10 | Rca Corporation | Dual-mode adaptive parameter processor for continuous wave radar ranging systems |
JPS6048710B2 (en) * | 1977-04-26 | 1985-10-29 | 住友金属工業株式会社 | Distance measurement method and device |
-
1981
- 1981-08-10 US US06/291,854 patent/US4435708A/en not_active Expired - Lifetime
-
1982
- 1982-08-09 FR FR8213848A patent/FR2511159A1/en active Granted
- 1982-08-10 JP JP57139092A patent/JPS5841372A/en active Pending
-
1990
- 1990-09-25 JP JP1990099066U patent/JPH0342388Y2/ja not_active Expired
Cited By (9)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4905009A (en) * | 1987-06-01 | 1990-02-27 | Kaman Aerospace Corporation | Distance measuring device |
EP0359911A2 (en) * | 1988-09-12 | 1990-03-28 | Daimler-Benz Aerospace Aktiengesellschaft | Radar altimeter |
US4945360A (en) * | 1988-09-12 | 1990-07-31 | Messerschmitt-Boelkow-Blohm Gmbh | Radar altimeter |
EP0359911A3 (en) * | 1988-09-12 | 1991-04-17 | Daimler-Benz Aerospace Aktiengesellschaft | Radar altimeter |
US5325095A (en) * | 1992-07-14 | 1994-06-28 | The United States Of America As Represented By The United States Department Of Energy | Stepped frequency ground penetrating radar |
US5869747A (en) * | 1996-05-22 | 1999-02-09 | William H. Hulsman | Food container internal pressure analysis |
US20080074308A1 (en) * | 2006-09-27 | 2008-03-27 | Honeywell International Inc. | Method and system of improving altimeter accuracy by use of a separate peak return signal tracking |
US7463187B2 (en) * | 2006-09-27 | 2008-12-09 | Honeywell International Inc. | Method and system of improving altimeter accuracy by use of a separate peak return signal tracking |
US20100245160A1 (en) * | 2009-03-31 | 2010-09-30 | Kabushiki Kaisha Toshiba | Fmcw signal generation circuit |
Also Published As
Publication number | Publication date |
---|---|
JPH0355581U (en) | 1991-05-29 |
FR2511159B1 (en) | 1984-12-21 |
JPS5841372A (en) | 1983-03-10 |
FR2511159A1 (en) | 1983-02-11 |
JPH0342388Y2 (en) | 1991-09-05 |
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